I was testing symbology to see what the complexity for lines are available now in the Map Viewer and Field Maps.
As none of the symbols I created in Pro first, worked in AGOL or in Field Maps I started testing the symbols in AGOL. They can be seen in the image below. These symbols also work in Field Maps.

I opened the WebMap in Pro and the symbology as expected, stayed as is as shown below.

I tried changing some of the colors and then saving the Pro Map as a Web Map using the share Save As Web Map tool.
When I opened the new map in Viewer, the symbology had downgraded.

Why is this occurring?